The Mysteries of Ship Tonnage

There’s a lot of misunderstanding over ship tonnage. Too often we read that such-and-such a ship, ‘weighed in at 80,000 gross tons’. Where the writer realises that gross tonnage is a measure of enclosed space and not of weight, it may still be written simply as, ‘80,000 gross tons. What’s wrong with that?’ Simply that today we now refer to ‘so many GT’ (or gt). It is correct to refer to ‘80,000 gross tonnage’; but not to ‘gross tons’.

Pity the poor landlubber! We seafarers, and our fellow conspirators, the naval architects, have really ‘set the cat amongst the pigeons’ and you may say that quibbling over such detail is petty. But a better understanding would help in getting it absolutely right.

What with gross tons, net tons, displacement tons, deadweight tons and freight tons (and a few more), how can the poor landlubber be expected to understand ship tonnage? And with our legislators introducing us to metric units a few years ago even some of our own brethren have become confused too.

Changes in recent years have altered the way we now consider the size of ships, but let’s start with the origin of the ‘ton’. It was originally a ‘tun’ referring to a barrel or cask used in the wine trade as a measure of quantity. By an Act of 1423 it was not less than 252 gallons, the weight of which approximated that of the unit which became known as the ton.

Things have changed over the years and especially in those countries which rejected the old Imperial system of weights and measures and adopted the metric system. Among other things it has lately meant a change for units of mass, from tons to tonnes.

Although many of us grew up with the ton, a unit of weight of 2,240 pounds, we readily accepted the change to a metric tonne of 1,000 kilograms, a slightly smaller unit of weight than the old ton. We used to refer to a weight of ‘so many tons’, but now refer to ‘so many tonnes’ although some of the older generation may occasionally forget.

Let’s now specifi cally consider the ‘tonnage’ of ships — a word which probably came into use for levying dues on wine-carrying vessels. However, as long as we are talking about weight there is not too much diffi culty with some types of ship tonnage which we will consider.

When we now refer to the weight of a ship, or of her cargo, (I emphasise we are here referring to weight) we now have to think in tonnes and not tons, making a mental note that the unit we are now using is slightly lighter than the unit used previously. This means that ships and cargoes now weigh slightly more tonnes than they used to weigh in tons. Otherwise there is generally not a problem.

Displacement tonnes A warship’s tonnage is usually stated in displacement tonnes which is her actual weight in metric tonnes as if measured on a giant set of scales. It gets its name from the fact that it is actually the weight of water she displaces when floating in sea water.

(Remember Archimedes and his cry ‘Eureka’ when he overflowed his bath?)

Light displacement A warship’s displacement tonnage can be either her ‘light displacement’ (the weight of the ship alone) or her ‘load displacement’ (complete with fuel, stores, ammunition and ship’s company) and it is necessary to state which it is.

Unlike naval ships the size of merchant ships is very rarely given as a displacement tonnage.

Deadweight tonnes Cargo ships, especially oil tankers, may have their size expressed in deadweight tonnes (DWT) and this is a measure of the maximum weight of cargo the ship can carry without illegally submerging her loadline when afloat in sea water.

Seafarers today remember with gratitude the work of Samuel Plimsoll who, as a Member of Parliament fought hard in the mid-19th century to prevent the overloading of ships then causing the loss of many seamen’s lives. (The principal loadline on a ship — a legal requirement —is to this day often known as her Plimsoll line and a master who overloads his ship breaks the law, and strong safeguards are now in place to ensure ships are not overloaded.)

Many years ago dues were based upon the number of tuns of wine that could be fitted into the ship and this provided a convenient, if not very accurate, measure of a ships size. Today much more accurate methods are used and ships are accurately measured internally to arrive at the ‘tonnage’ of passenger and cargo ships.

Read the rest of this article with additional pictures in Sea Breezes Magazine - June 2016 Issue Click here to subscribe